The ever-pressing requirements to increase the productivity of machine tools, to improve the quality of the mechanical pieces that these machine tools produce, on the basis of ever-tighter tolerances, and to diminish the production costs call for minimizing the machine down-times and concurrently performing, whenever necessary, all the maintenance operations required for avoiding both the down-grade of the quality of the produced pieces and the costs deriving from scraps and re-machining of pieces.
For checking the machining process of computer numerical control (“CNC”) machine tools, as lathes, grinding machines, milling machines, etc. there are utilized apparatuses provided with sensors that detect the magnitude of physical features connected to the process to be checked and indicate, to the machine numerical control, or directly to the operator, the need to perform maintenance or corrective procedures.
Similar apparatuses are those, for example, that detect the wear or the breakage of the tools and signal the need for their substitution or sharpening.
U.S. Pat. No. 5,070,655 discloses an apparatus for monitoring some specific working conditions of a machine tool, more particularly for detecting grinding wheel sharpness, loss of coolant or excessive vibrations.
The apparatus processes signals indicative of the power consumption, as provided by a power-detecting sensor, and signals indicative of the mechanical vibrations, as provided by an accelerometer, and emits a visual signal (in the form of a green, a yellow or a red light, the latter is associated with an acoustic signal) for indicating to the operator the condition of the process under control.
U.S. Pat. No. 5,718,617 discloses a system for measuring a force existing between a grinding wheel and a piece in the course of the machining of a computer numerical control (“CNC”) grinding machine, by a force-detecting sensor mounted between the ball screw that activates the tool-carrier slide and the machine base. In the machine there can also be mounted accelerometers for compensating the signals arriving from the force-detecting sensor by removing components of said signals that depend on vibrations.
U.S. Pat. No. 5,044,125 discloses a machine tool, more specifically a grinding machine, with a force transducer that comprises piezoelectric sensors mounted adjacent the wheelhead to measure the magnitude of the force occurring between the grinding wheel and the piece. The signals output by the force transducer are sent to the numerical control that determines when there is the need to perform a grinding wheel dressing cycle.
The signal output by the force transducer is suitably processed and provides significant information relating to the machining or to the grinding wheel sharpness, but does not enable to accurately distinguish the time intervals when the grinding wheel is actually in contact with the piece. In fact, the amplitude of the signal output by the force transducer slowly increases when contact between grinding wheel and piece occurs and it slowly decreases when the grinding wheel is detached from the surface of the piece, as hereinafter described. Furthermore, immediately after the displacing of the grinding wheel away from the piece, the output signal of the force transducer has spurious components, due to the displacing of parts of the machine tool, that can have an amplitude comparable with that of the signal in the course of the machining and must not be considered during the processing.